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locking/barriers: Introduce smp_acquire__after_ctrl_dep()

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Introduce smp_acquire__after_ctrl_dep(), this construct is not
uncommon, but the lack of this barrier is.

Use it to better express smp_rmb() uses in WRITE_ONCE(), the IPC
semaphore code and the qspinlock code.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2016-05-24 13:17:12 +02:00 committed by Ingo Molnar
parent 1f03e8d291
commit 33ac279677
3 changed files with 15 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
include/linux/compiler.h
View File

@ -304,6 +304,17 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
__u.__val; \
})
/**
* smp_acquire__after_ctrl_dep() - Provide ACQUIRE ordering after a control dependency
*
* A control dependency provides a LOAD->STORE order, the additional RMB
* provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
* aka. (load)-ACQUIRE.
*
* Architectures that do not do load speculation can have this be barrier().
*/
#define smp_acquire__after_ctrl_dep() smp_rmb()
/**
* smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
* @ptr: pointer to the variable to wait on
@ -314,10 +325,6 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
*
* Due to C lacking lambda expressions we load the value of *ptr into a
* pre-named variable @VAL to be used in @cond.
*
* The control dependency provides a LOAD->STORE order, the additional RMB
* provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
* aka. ACQUIRE.
*/
#ifndef smp_cond_load_acquire
#define smp_cond_load_acquire(ptr, cond_expr) ({ \
@ -329,7 +336,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
break; \
cpu_relax(); \
} \
smp_rmb(); /* ctrl + rmb := acquire */ \
smp_acquire__after_ctrl_dep(); \
VAL; \
})
#endif

14
ipc/sem.c
View File

@ -259,16 +259,6 @@ static void sem_rcu_free(struct rcu_head *head)
ipc_rcu_free(head);
}
/*
* spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
* are only control barriers.
* The code must pair with spin_unlock(&sem->lock) or
* spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient.
*
* smp_rmb() is sufficient, as writes cannot pass the control barrier.
*/
#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb()
/*
* Wait until all currently ongoing simple ops have completed.
* Caller must own sem_perm.lock.
@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_array *sma)
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
}
ipc_smp_acquire__after_spin_is_unlocked();
smp_acquire__after_ctrl_dep();
}
/*
@ -350,7 +340,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
* complex_count++;
* spin_unlock(sem_perm.lock);
*/
ipc_smp_acquire__after_spin_is_unlocked();
smp_acquire__after_ctrl_dep();
/*
* Now repeat the test of complex_count:

2
kernel/locking/qspinlock.c
View File

@ -379,7 +379,7 @@ void queued_spin_unlock_wait(struct qspinlock *lock)
cpu_relax();
done:
smp_rmb(); /* CTRL + RMB -> ACQUIRE */
smp_acquire__after_ctrl_dep();
}
EXPORT_SYMBOL(queued_spin_unlock_wait);
#endif